Two Phase Cooling
Passive two-phase heat transfer devices come in a variety of constructions, but operate under the same key principal of utilizing vapor to transfer heat quickly and efficiently to where it can be more effectively dissipated. These are closed systems that contain a small amount of liquid that is then vacuum sealed. Heat is absorbed from the heat source where it evaporates the local liquid. The vapor then travels to a cooler region of the solution where it cools and condenses back into a liquid state. The liquid then returns to the evaporator portion through either gravity or capillary action.
As there are no moving parts, this cycle can repeat indefinitely. Additionally, because of the latent heat of vaporization, fluid can absorb and carry more energy or heat during phase change from liquid to a vapor. This provides a much higher effective conductivity than the metals typically used for heat sinks or single phase cooling such as a liquid or air cooled system.
Because of their ability to passively transfer high heat loads, two-phase cooling devices such as heat pipes and vapor chambers are often integrated into cooling solutions and systems to further optimize performance.
Heat Pipes offer a significant amount of design flexibility as they can be used for heat spreading within the base of a heat sink or for heat transfer to remote fin stacks or cooling systems. Most commonly constructed of copper and water, they can also be made utilizing other fluids and materials for specific application needs. Sintered powder wicks are most often used to enable the capillary action that allows the fluid to return to the evaporation point without the use of gravity; however other wick structures can be utilized based on design requirements.
Vapor chambers work in much the same fashion as heat pipes, however they have a planar construction that allows heat to spread in more directions than the more axial heat pipes. Because of this, they are often used for heat spreading within a heat sink base to greatly improve performance.
Unlike heat pipes and vapor chambers, thermosiphons use gravity to return the liquid to the evaporator and therefore have more design considerations regarding orientation. Assemblies typically include integrated fins as part of the actual thermosiphon and are often on a larger scale than heat pipes or vapor chambers as it is a more complete system. When reliability is a major factor, thermosiphons can be considered as a potential replacement for active liquid cooling.